This invention relates to acoustic structures for interaction with surface acoustic waves, and more particularly to such structures suitable for arrays of acoustic reflectors in acoustic touch sensors and to organic matrix materials for making such structures.
"Surface acoustic waves" (SAW), as used herein, refers to acoustic waves for which a touch on the surface leads to a measurable attenuation of acoustic energy. Several types of surface acoustic waves are known. The vast majority of present commercial products are based on Rayleigh waves, which maintain a useful power density at the touch surface because they are bound to the touch surface. Mathematically, Rayleigh waves exist only in semi-infinite media. In practice it is sufficient for the substrate to be 3 or 4 wavelengths in thickness, leading to quasi-Rayleigh waves that are practical equivalents of true Rayleigh waves. Herein, it is understood that Rayleigh waves exist only in theory and a reference thereto indicates a quasi-Rayleigh wave. Like Rayleigh waves, Love waves are surface-bound waves. While Rayleigh waves have vertical and longitudinal particle motion and both shear and pressure/tension stresses associated therewith, Love waves have only horizontal particle motion (parallel to touch surface) and only shear stress associated therewith. Other surface-bound waves are known.
Another class of surface acoustic waves relevant to acoustic touchscreens are plate waves. Unlike surface-bound waves, plate waves require the confining effects of both the top and bottom surfaces of a substrate to maintain a useful power density at the touch surface. Examples of plate waves include symmetric and anti-symmetric Lamb waves, zeroth order horizontally polarized shear (ZOHPS) waves, and higher order horizontally polarized shear (HOHPS) waves.
SAW devices are used as touch sensors, signal filters, and in other applications. A common touch sensor design has two sets of transducers, each set having a different axis aligned respectively with the axes of a physical Cartesian coordinate system defined by a substrate. An acoustic pulse or pulse train is generated by one transducer, propagating as a Rayleigh wave along an axis which intersects an array of reflective elements, each element angled at 45.degree. and spaced corresponding to an integral number of acoustic wavelengths. Each reflective element reflects a portion of the wave along a path perpendicular to the axis, across an active region of the substrate, to an opposing array and transducer which is a mirror image of the first array and transducer. The transducer in the mirror image array receives an acoustic wave consisting of superposed portions of the wave reflected by the reflective elements of both arrays, directed antiparallel to the emitted pulse. Wavepaths in the active region of the sensor have characteristic time delays, and therefore any wavepath attenuated by an object touching the active region may be identified by determining a timing of an attenuation in the composite returning waveform. A second set of arrays and transducers are provided at right angles to the first, and operate similarly. Since the axis of a transducer corresponds to a physical coordinate axis of the substrate, the timing of an attenuation in the returning wave is indicative of a Cartesian coordinate of a position on the substrate, and the coordinates are determined sequentially to determine the two dimensional Cartesian coordinate position of the attenuating object. Other acoustic touch position sensor designs may be used. Illustrative designs are disclosed in U.S. Re Pat. No. 33,151; U.S. Pat. No. 4,642,423; U.S. Pat. No. 4,644,100; U.S. Pat. No. 4,645,8870; U.S. Pat. No. 4,700,176; U.S. Pat. No. 4,746,914; U.S. Pat. No. 4,791,416; U.S. Pat. No. 4,825,212; U.S. Pat. No. 4,880,665; U.S. Pat. No. 5,072,427; U.S. Pat. No. 5,162,618; U.S. Pat. No. 5,177,327; U.S. Pat. No. 5,234,148; U.S. Pat. No. 5,260,521; U.S. Pat. No. 5,260,521; U.S. Pat. No. 5,329,070; and U.S. Pat. No. 5,451,723; all incorporated herein by reference.
The maximum acoustic path length traveled by the acoustic pulse is a useful metric in acoustic touchscreen design, because most materials, e.g., glass, have a relatively constant acoustic power loss (dB per unit length): the greater the path length, the greater the attenuation. In many cases, this attenuation limits the design of the touchscreen. Therefore it is generally desirable to have high acoustic efficiency. Thus, for example, greater numbers of transducers may be deployed to allow larger substrates, and likewise, with smaller substrates, acoustic paths may be folded to reduce the number of transducers.
The array of reflective elements forms a critical part in the design of an acoustic touchscreen. Because the elements are placed along the path of the acoustic wave, it is generally desired that the ratio of reflection to absorptivity be maximized, allowing sufficient wave energy to reach the end of the array while reflecting sufficient wave energy to allow reliable detection of the touch.
Generally, SAW propagation efficiency is maximized by the wave's interaction with a brittle material such as glass and avoiding materials which appear viscous to it. In fact, it is the inefficient propagation of the surface acoustic wave due to interactions with environmental effects which enables SAW-based chemical or humidity sensing by measuring the signal loss due to the interactions.
Present commercial acoustic touchscreens typically are built from soda-lime glass and placed immediately in front of a display device such as a cathode ray tube (CRT), with the reflective arrays disposed at the periphery of the substrate, outside of the active sensing area and hidden and protected under a bezel. The reflective elements each generally reflect of order 1% of the SAW power, dissipating a small amount and allowing the remainder to pass along the axis of the array. Thus, array elements closer to the transmitting transducer will be subjected to greater incident acoustic energy and reflect a greater amount of acoustic power. In order to provide equalized acoustic power at the receiving transducer, the spacing of the reflective elements may be decreased with increasing distance from the transmitting transducer, or the acoustic reflectivity of the reflective elements may be altered, allowing increased reflectivity with increasing distance from the transmitting transducer.
An acoustic beam incident on a reflector produces a transmitted (unreflected) portion, a reflected portion, and an absorbed portion. An optimal reflector material is one with minimal acoustic absorption. More precisely it is desirable to maximize the ratio of reflected power to absorbed power. The mass of material deposited can be adjusted to achieve a desired ratio of reflected to transmitted power.
Present reflective arrays are generally a chevron pattern of raised glass frit interruptions on a soda-lime glass substrate. The interruptions typically have heights or depths on the order of 1% of the acoustic wavelength, and therefore only partially reflect the acoustic energy. Glass frits such as those in touchscreens from Elo TouchSystems, Inc., Fremont, Calif., are near optimal in this regard. When cured (fused), they have a high mechanical quality factor or Q, which is a measure of quality and freedom from internal damping, or more technically, the quotient of the resonant frequency and the bandwidth. This leads to minimal acoustic absorption by the reflectors.
Glass frit is generally supplied as a printable gel-like ink comprising a mixture of solvents, organic binders imparting thixotropic properties, and fine particles of "solder glass" (a mixture of zinc and lead oxides). The glass frit is screen-printed onto the substrate and transformed into a hard glassy substance by a high temperature (over 400.degree. C.) cure in an oven to evaporate the solvents, burn off the binders, and sinter together the remaining fine particles of solder glass. The substrate carrying the printed reflector pattern is necessarily subjected to the same high temperature during sintering. Thus, the choice of substrates is restricted: Tempered glass looses its temper at such curing temperatures and CRT faceplate glass cannot be used because of the heat sensitivity of the CRT's other components. Another limitation is the presence of heavy metals whose potential for leaching out by, e.g., the acetic acid found in some glass cleaners is a health hazard.
The glass frit cure process is a significant cost component. Ovens are a major capital expense and consume significant amounts of electric power. Conveyor ovens require considerable floor space, while batch ovens disrupt a smooth manufacturing flow. The cure process takes time, about one hour for a conveyor oven and about eight hours for a batch oven. Thus, it is desirable to supplement or replace the glass-frit process.
Polymers tend to be more acoustically absorptive than glass, and even a small amount thereof deposited as reflectors on a touchscreen can cause significant acoustic attenuation. Therefore, in comparison with glass frit systems, the ratio of the transmitted signal to the minimally acceptable received signal amplitude will be less for polymers, typically exceeding 6 dB in a 14 inch touchscreen.
In addition to their significant acoustic attenuation properties, many polymers are hydrophilic. An epoxy may absorb up to 10% moisture. This absorption may result in delamination of an epoxy film from a substrate such as glass. Thus, many epoxies have poor bonding ability to glass under a range of environmental conditions.
Epoxies have been used as absorbers of acoustic waves. U.S. Pat. No. 4,090,153 teaches filled epoxy resin as an absorber of Rayleigh waves propagating on the surface of a piezoelectric substrate. U.S. Pat. No. 4,510,410 relates to filled ultraviolet (UV) light curable resin acoustic wave absorbers for piezoelectric substrates. The patent presents data (FIG. 3) showing that the actual absorption rises with increasing specific gravity (filler loading) and considers reflections off the absorbers to be parasitic. U.S. Pat. No. 5,400,788 relates to a tungsten filled epoxy employed as an absorber of pressure waves. U.S. Pat. No. 5,488,955 relates to a tungsten filled epoxy employed as a beam dump (a structure for diverting and eliminating beam energy from a normal acoustic beam path). While a portion of the acoustic waves may be reflected, in this case the goal is to attenuate as much of the wave as possible. The preceding four patents are incorporated herein by reference.
U.S. Pat. Nos. 5,113,115 and 5,138,215, incorporated herein by reference, relate to unfilled polymer acoustic reflectors, making similar use of polymer Rayleigh-wave reflectors on a piezoelectric substrate, for diverting beam energy from the active acoustic path. U.S. Pat. No. 5,138,215 describes a transmit reflective array, a receive reflective array, as well as the polymer array which serves as a "beam dump" array. The beam-dump array is referred to as the "third" or "auxiliary" array. The patent does not teach that the transmit or receive arrays may be formed of polymer. In a beam dump system, the goal is to make the absorption of acoustic waves efficient, while preventing the introduction of parasitic waves. In sharp contrast to the transmit and receive arrays, the ability of the beam-dump array to transmit an acoustic beam down its axis is not of particular concern.
U.S. Pat. No. 5,260,913 discloses encasing SAW devices to shield them from environmental influences. Low modulus potting materials absorb Rayleigh waves, so wave modes other than Rayleigh, such as horizontal shear waves, are employed. "Acoustic Properties of Particle/Polymer Composites for Transducer Backing Applications" by Grewe et al. from 1989 Ultrasonics Symposium, and "Ultrasonic Measurement of Some Mineral Filled Plastics" by Lees et al., IEEE Transactions on Sonics and Ultrasonics, Vol. SU-24, No. 3, May 1977, disclose using tungsten loaded epoxies for acoustic purposes. However, these references do not suggest the use of such materials for reflective arrays or printable inks.
As is evident from the above discussion, polymeric acoustic reflectors are desirable for acoustic touchscreens in which the substrate is tempered glass, the faceplate of a CRT, or another substrate which cannot tolerate the high cure temperatures of glass frits. However, the use of polymers poses serious problems for the design engineer, being limited by the polymer's (a) adhesion to a suitable substrate; (b) acoustic absorptivity; (c) mechanical stability toward a range of environmental conditions; and (d) acoustic property stability over a range of environmental conditions; and (e) availability of a process for fabrication into quality reflective elements. These factors have proven sufficiently complex that extant systems have continued to employ glass frits on soda lime glass substrates.